ASME CRTD-103-2014 pdf download
ASME CRTD-103-2014 pdf download.CONSENSUs ON BEST TUBE SAMPLING PRACTICES FOR BOILERS NONNUCLEAR STEAM GENERATORS.
HP Evaporator Tubes in Horizontal Gas Path (Vertical Tube) HRSG. If the units have duct burners, target 1/3 of the distance from the wall with the gas burner supply. For vertical tube evaporators, the following elevations have been recommended by three different HRSG suppliers:
1. Two feet (about 0.6 meter) above the bottom duct burner,
2. In the center of the tubes, and
3. Near the tops of the tubes (location of highest steam quality).
For Horizontal Gas Path HRSG Units, it can be difficult to collect a sample at the upper two elevations (items 2 and 3 listed above) during the typically short outages used at HRSG plants. Collecting a sample closer to the bottom (as high as can be reasonably removed without scaffolding or at 2 feet (about 0.6 m) above the bottom duct burner) may not represent the location with the highest deposit weight (worst case). However, these samples can provide useful information to indicate whether deposit accumulation may be a problem in the unit. Therefore, this was the elevation included in Table 1. This information can be used to justify more extensive scaffolding of the furnace in a future outage.
Some owners select outer lane tubes if there are indications of gas bypassing the upstream superheaters. If the unit has experienced plugging of the gas pass or loss of baffles, one should focus on areas expected to have the highest flue gas flow rates. Typically, the first row tubes are sampled. While it has been common to expect the first row tubes to be those experiencing the highest heat flux, one also has to consider the amount of fins applied to the tube. In some cases a second row tube with more fins may absorb more heat than a first row tube without fins.
For Vertical Gas Path HRSG Units (with Horizontal Tubes). The greatest deposits can be at the point at which steam first starts to form. Consult the HRSG manufacturer regarding suitable sample locations.
Failed Tubes in Boilers and HRSG Units. It usually is best not to select a tube that has experienced a failure, especially a failure that results in deformation of the metal (such as bulging or rupture) for deposit weight analysis. This can result in spalling of the deposits from the tube surface and result in a lower deposit weight than actually present. Samples for deposit weight analysis should preferentially be removed from tubes next to those that have experienced a failure. However, failed tubes should also be sampled and analyzed to determine the failure mechanism.
If only the failed tube is sampled, a sufficient tube length should be removed to assess the deposition on both sides of the failure. In some units, circulation problems can result in an adjacent tube having vastly less deposit and a deposit weight assessment is needed on both the failed tube and the adjacent tube. Flow disruptions can occur downstream from circumferential welds used for tube replacement, and this typically results in higher localized deposit loading values. If a tube is sampled that contains a circumferential weld that protrudes from the internal surface of the tube, then remove a section that extends at least 2 feet (about 0.6 rn) from the weld on either side. As a boiler is only as clean as the dirtiest area, the weld location is a valid sample. Sometimes the deposit weight is determined both at the weld and away from the weld to characterize average and localized deposit accumulations.
Sampling Newer Tubes. Tubes that have been repaired or replaced since the last chemical cleaning will not be representative of the heaviest deposits, but can be used to indicate deposit accumulation since the tube was installed. This can be useful when there have been chemistry upsets or major changes in the treatment program.
